This invention relates generally to optical communication systems and more particularly to optical laser sources with multiple lasing wavelengths.
One way to utilize the large bandwidth of optical fibers is to use optical wavelength division multiplexing (WDM) schemes to increase the rate of data transmission through optical fibers. In the transmitter end of a WDM transmission system and network it is necessary to have number of laser sources with different wavelengths. Each laser light is then modulated either directly in the laser or by an external modulator to impress the data information on each of the WDM channels.
Multi-wavelength laser sources are desirable for optical wavelength division multiplexed (WDM) transmission systems and networks.
U.S. Pat. No. 5,910,962 introduces a multi-wavelength laser source which provides multiple laser signals at different wavelengths incorporating DBR (Distributed Bragg Reflector) fiber lasers or DFB (Distributed Feedback) fiber lasers. In the proposed design, a pumping laser with operating wavelength below 1500 nm is used. Each DFB or DBR laser is tuned to a selected wavelength in the 1550 nm range. The fiber lasers may be connected in series to each other to form a multi-wavelength source. There are a number of issues with respect to this arrangement. We can easily observe that the number of wavelength channels is very limited. As a result, it is very difficult to scale the source to have a large number of lasing channels. On the other hand, the processes of tuning each laser and consequently the whole set of wavelength channels are very challenging. Stability of the lasing frequencies is also of great concern.
M. Zirngibl et at. in xe2x80x9cAn 18 channel Multi-Frequency Laser,xe2x80x9d IEEE Photonics Technology Letter, Vol. 8, No. 7 July 1996 propose an array of semi-conductor optical amplifiers integrated monolithically with a ADM multiplexers/demultiplexers. In this architecture, a Wavelength Grating Router (WGR) is used as an intra-cavity wavelength selective filter element. The number of amplifiers and the complexity of the WDM filter increase as the number of lasing channels or, equivalently, as the channel spacing in a given wavelength range decreases. The wavelength selectivity of this device is governed by the geometric layout of the filter and a discrete choice of the gain elements. As a result, for a large number of wavelength channels, a large number of router arms are needed. The cost of these multi-wavelength laser sources is therefore, high and they cannot be easily fabricated for a large number of wavelengths or lasing channels. Similar designs have also been reported, such as R. Monnard et al. in xe2x80x9cDirect Modulation of a Multi-frequency Laser up to 16xc3x97622 Mb/s,xe2x80x9d IEEE Photonics Technology Letters, Vol. 9, No. 6, June 1997.
It is therefore desirable to have a low cost multi-wavelength laser source with a large number of lasing channels.
In the present invention an optical gain element or medium is used where in a fraction of the optical output signal is passed through a periodic filter and fed back to the gain medium. This configuration simply forms a multi-wavelength ringer laser. The optical gain element provides the gain medium for the laser and the filter forces the laser to lase on the predetermined wavelengths. The periodic filter may simply be an asymmetric Mach-Zehnder Interferometer (MZI). It is known that asymmetric MZIs have an almost sinusoidal wavelength response, where the period is function of the length difference between the arms of the asymmetric MZI. In other words, one can control the channel spacing by changing the length difference of an asymmetric MZI.